Calcium isotope fractionation during dolomite formation

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Author(s): Böhm, Florian; Eisenhauer, Anton; Fietzke, Jan; Rausch, Svenja; Klügel, Andreas; Bach, Wolfgang
Author Affiliation(s): Primary:
Leibniz Institute of Marine Sciences, GEOMAR, Kiel, Germany
Other:
University of Bremen, Germany
Volume Title: Goldschmidt 2011 abstract volume
Source: Mineralogical Magazine, 75(3), p.544; Goldschmidt2011, Prague, Czech Republic, Aug. 8-14, 2011. Publisher: Mineralogical Society, London, United Kingdom. ISSN: 0026-461X
Note: In English. 5 refs.
Summary: Dolomite is an important component of the global calcium cycle, being a source of Ca to the oceans [1]. Calcium released to the oceans by dolomitization of chalk and limestones can potentially be quantified using marine Ca isotope records [2]. However, little is known about Ca isotope fractionation and the Ca isotopic composition of dolomitic rocks. Theoretical calculations point to a -2 ppm depletion in δ44/40Ca of dolomite relative to calcite at 25°C [3]. Ordovician dolostones interbedded with limestones were reported to be depleted in δ44/40Ca by -0.6 ppm relative to the limestones [4]. We have investigated dolomites from an ODP core (Site 183-1140) drilled at the Northern Kerguelen Plateau (46.3°S 68.5°E, 2394 mbsl). The core penetrated 235 m of nannofossil ooze and chalk of early Oligocene to middle Miocene age, and 88 m of pillow basalts forming the basement for the sediments. The basalts erupted at about 34 Ma (latest Eocene) [5]. An interbedded chalk layer was found in the basalt, about 40 m below the top of the basement. The chalk was partly dolomitized at the contact with the basalt. We measured oxygen, carbon, calcium and radiogenic strontium isotopes of bulk chalk and dolomite samples. The 87Sr/86Sr ratios indicate an age of dolomitization of about 10 Ma (late Miocene). Dolomitization obviously occurred about 20 Ma after eruption of the basalts, in a deep burial setting. Oxygen isotope values of chalk and dolomite demonstrate that dolomitization occurred at slightly elevated temperatures (10-20°C). The calcium isotopes of the dolomite are enriched in δ44/40Ca by about +0.5 ppm compared to the chalk. This is in contrast to the depletion of δ44/40Ca in dolomite reported in the literature. On the other hand, dolomite veins in ocean crust basalts (DSDP/ODP Sites 37-332 and 129-801) are depleted in δ44/40Ca compared to calcite veins of similar age. Fluid composition, diagenetic history and kinetic isotope fractionation have to be considered when interpreting Ca isotope values of dolomite.
Year of Publication: 2011
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 06 Petrology, Sedimentary; Alkaline earth metals; Basalts; Ca-44/Ca-40; Calcium; Carbon; Carbonate rocks; Carbonates; Carbonatization; Cenozoic; Chalk; Clastic sediments; Cores; Crust; Crystal growth; Dolomite; Dolomitization; Experimental studies; Geochemical cycle; Geochemistry; Igneous rocks; Indian Ocean; Isotope fractionation; Isotope ratios; Isotopes; Kerguelen Plateau; Lava; Leg 183; Metals; Miocene; Neogene; ODP Site 1140; Ocean Drilling Program; Oceanic crust; Oligocene; Ooze; Oxygen; Paleo-oceanography; Paleogene; Pillow lava; Sedimentary rocks; Sediments; Sr-87/Sr-86; Stable isotopes; Strontium; Tertiary; Volcanic rocks
Coordinates: S461536 S461536 E0682930 E0682930
Record ID: 2012064475
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